1. Field of Invention
The present invention relates to a controlling method, and more particularly to a method for controlling a fan.
2. Description of Related Art
Along with the progress in science and technology, electronic devices such as notebooks or desk-top computers have been widely used. In general, to prevent the waste heat generated by the electronic devices during operations from affecting the performance thereof, fans are usually disposed to dissipate the waste heat. Further, the current techniques can flexibly adjust the heat dissipation rate via directly modulating the rotational speed of the fan by software programs according to the temperature of the electronic devices.
FIG. 1 is a schematic view of a conventional method for controlling the rotational speed of a fan, wherein the horizontal coordinate represents time and the vertical coordinate represents the rotational speed of the fan. Referring to FIG. 1, the fan has low speed, intermediate speed and high speed mode values for rotational speed, which respectively correspond to 2000, 3000 and 4000 rpm. The rotational speed of the fan is determined by the input voltage of the fan. At first, the fan is rotated at a rotational speed of low speed mode value (2000 rpm), and at a time point t11, the rotational speed of the fan is adjusted to the intermediate speed mode value (3000 rpm) for raising the waste heat dissipation rate. After that, at a time point t13, the rotational speed of the fan is adjusted to the high speed mode value (4000 rpm) for dissipating the waste heat at a full speed. The requirements of heat dissipation at different time periods can be satisfied by adjusting the rotational speed of the fan.
In view of the above, the fan sounds in operation. When the fan rotates faster and faster, the decibel (dB) and frequency of the sound from the fan also become higher. Seen from acoustics researches, human considers a continuous single pure tone as a noise. Therefore, as the rotational speed of the fan remains for a period of time at the low speed, intermediate speed and high speed mode values respectively, the sound made by the fan in each of the modes is a continuous single pure tone and thus is considered as a noise by the user. More particularly, when the fan rotates faster, the sound of a high dB and high frequency made by the fan may be considered as a greater noise by the user.
Therefore, when the rotational speed of the fan is switched, for example, at the time point t11, the rotational speed of the fan is adjusted from the low speed mode value to the intermediate speed mode value, the dB and frequency of the single pure tone (noise) made by the fan are changed. As such, the user may feel sick by sensing the noise. Especially, during the period from t11 to t12, the user is still paying attention to the single pure tone (noise) after the mode switch, so the user may have a worse feeling for the noise.